The invention relates to methods for lowering the nitrogen oxides in flue gas produced from the combustion of substantially any combustible fuel including solid fuels, sludges, gaseous fuels or the like. However, the invention provides a particularly improved fluidized bed combustion process wherein the effluent stack gases can be managed economically to meet current environmental standards.
Fluidized bed combustion of fuels is a well known practice. Typically, air is introduced through a plenum where it is distributed through an air distribution grid. Fuel fluidizing particles and sorbents, such as limestone or dolomite, are fluidized and reacted in the furnace at temperatures normally in the 1400.degree.-1700.degree. F. range. This temperature is substantially lower than those practiced with a conventional furnace. This temperature range, besides resulting in excellent fuel burnout, is also suitable for reacting sulfur oxides with the sorbents in the combustion chamber. Thus, sulfur oxide emissions from sulfur-containing fuel can be substantially reduced, as by the addition of limestone, allowing the burning of relatively high sulfur coals with reduced pollution.
Nitrogen oxides are generated when burning any fuels and result from thermal fixation of nitrogen in the air and the conversion of fuel nitrogen. The former reaction is favored at high temperatures (above about 1800.degree. F.) while the latter is of greater concern at lower temperatures, e.g., those generally found in fluidized bed combustion systems. Because nitrogen oxides are related to the formation of "photochemical smog" and can be poisonous at low exposure levels (the TLV of NO.sub.2 is 5 PPM), there is an ongoing concern with the minimization of the NO.sub.x levels released from combustion systems.
It has been suggested in U.S. Pat. No. 3,900,554 to non-catalytically remove nitrogen oxides from flue gases having exited a conventional furnace by injecting ammonia into the effluent stream while it is at a temperature range of 1600.degree.-2000.degree. F. European published patent application No. 176,293 also discloses the use of NH.sub.3 for NO.sub.x control via injection into a flue gas stream prior to its entry into a centrifugal separator. U.S. Pat. No. 4,335,084 suggests even higher temperatures. Many other patents have suggested the use of ammonia with catalysts to reduce nitrogen oxides. Some of these patents that utilize lower temperatures (e.g. 250.degree.-930.degree. F.) include U.S. Pat. Nos. 3,887,683 (activated charcoal catalyst), 4,056,660 (V.sub.2 O.sub.5 /Mn.sub.2 O.sub.3 catalyst), 4,010,238 (various transition metal catalysts), 4,002,723 (noble metal catalysts), 4,049,777 (CrO catalyst), 4,031,185 (Cu-halide catalyst) and 4,070,440 (alpha Fe.sub.2 O.sub.3 catalyst).
Several other U.S. patents for example, U.S. Pat. Nos. 3,894,141 and 3,867,507, suggest using a hydrocarbon rather than ammonia in order to reduce nitrogen oxides. Still other U.S. patents, such as U.S. Pat. Nos. 4,325,924 and 4,208,386, utilize urea for NO.sub.x emission reduction, and U.S. Pat. Nos. 4,154,803 and 4,507,269 disclose other ammonia precursors. Other U.S. patents, such as U.S. Pat. Nos. 4,119,702 and 4,115,515, utilize additives such as hydrogen, ozone and hydrogen peroxide to improve system performance.
U.S. Pat. No. 4,218,427 suggests using a fluidized bed of pulverized coal at a temperature from about 400.degree.-700.degree. C. U.S. Pat. No. 4,181,705 discloses the injection of ammonia or an ammonia-producing precursor directly into the fluidized bed combustion region of the furnace. U.S. Pat. No. 3,929,967 discloses a method for treating flue gases containing NO.sub.x and SO.sub.x primarily for reducing the amount of SO.sub.x by contacting the effluent gas with ammonia in gaseous form at a temperature of preferably about 700.degree.-800.degree. F. in an amount sufficient to react with substantially all of the sulfur trioxide; the reaction desirably results in the conversion of SO.sub.x to ammonium sulfite and ammonium bisulfate, following which reaction the larger solids are removed by a mechanical separator, such as a cyclone separator, followed by a high-temperature electrostatic precipitator. Subsequently ammonia is recovered from the ammonium sulfur oxide solids.
Many other processes are taught in the art for the removal primarily of sulfur dioxide, for example U.S. Pat. No. 4,369,167 teaches the use of a lime solution, which may also include a second scrubbing unit for specifically removing NO.sub.x using a suitable scrubbing medium, such as a dichromate. Of course it is well known to inject limestone into the combustion chamber itself for the reduction of SO.sub.x. It is also known that the use of ammonia for the treatment of flue gases, particularly at certain temperatures, will result in the removal of SO.sub.3 by reaction with ammonia and water to yield ammonium sulfate; thus, the use of ammonia or an ammonia precursor can also have an effect on reducing the SO.sub.x level.